Crystal structure of the nitrogenase iron protein from Azotobacter vinelandii

Abstract

The nitrogenase enzyme system consists of two component proteins, an iron (Fe-) protein and a molybdenum-iron (MoFe)-protein. Prior to substrate reduction, electrons are transferred from Fe-protein to MoFe-protein in an ATP-dependent process. The coupling of ATP hydrolysis to electron transfer is mediated by Fe-protein, which is the only known reductant of MoFe-protein supporting catalytic activity (8). Fe-protein is a dimer of two identical 32kDa subunits, the sequences of which are highly conserved in both conventional and alternate nitrogenases of a diverse group of prokaryotes. The Fe-protein dimer symmetrically binds one 4Fe-4S cluster to Cys 97 and Cys 132 from each subunit (sequence numbering of the Azotobacter vinelandii Fe-protein) (2,4). Unusual aspects of Fe-protein include the ATP-electron transfer coupling and the unique cluster environment, which is unlike any observed in ferredoxin structures. To address the structural basis of these properties, we have determined the crystal structure of the A. vinelandii Fe-protein at 3.0Å resolution. At present, 247 of the 289 residues have been modeled as a poly-alanine chain, and detailed fitting of the actual side chains is in progress. The current model is sufficient to begin addressing questions concerning the overall shape and fold of the protein, the environment of the cluster, the probable nucleotide binding site, the site(s) of interaction with MoFe-protein, and the interactions at the dimer interface.